In recent years, digitalization of cordless telephones used in houses has been developed, so that high-sound quality and improvements in the security of cordless telephones are achieved. In addition, portable handsets of cordless phones having not only a voice call function but also serving as a sensor or a surveillance camera have been introduced.
Meanwhile, highly functional cellular phones such as smartphones provided with advanced functions in addition to a communication function for communicating with a public mobile phone network have been developed. The advanced functions include: a Bluetooth (registered trademark) communication function for performing near field radio communication; or a global positioning system (GPS) receiver receiving signals from a GPS, for example.
When voice or video communication or data is made shareable by building a link between a cordless telephone disposed inside a house and a cellular phone, the base unit of the cordless telephone and the cellular phone need to be synchronized with each other so as to be capable of communicating with each other at any time. However, when the cellular phone is taken out to the outside of the house, the communication function of the cellular phone with the base unit does not need to be operated. Even when various communication functions are built in the cellular phone, putting these functions in operation always increases power consumption.
For example, Japanese Patent Application Laid-Open No. 2004-343608 discloses a radio phone device that avoids increases in power consumption in the case of being out of a service range. According to the disclosure, when it is detected that the radio phone device is out of a service range in which electric waves transmitted from the base unit in the house are receivable and when this state of the radio phone device continues at least for a predetermined time, the radio phone device stops a search for the base unit (open search), thereby avoiding increases in the power consumption when the radio phone device is out of the service range. In addition, it is possible to resume the search for the base unit as appropriate by being triggered upon an operation performed in an operation section or the like and to return the radio phone device to the communicable state.
In addition, the GPS receiver provided in the cellular phone is operated mainly when the cellular phone is taken out to the outside of the house. Thus, when the cellular phone is placed inside the house, the need for operating the GPS receiver is low. Since the GPS receiver continuously receives signals transmitted from satellites, the power consumption required for calculating the location of the GPS receiver is large. Accordingly, when the GPS receiver continuously operates even at an unnecessary location, the consumption of the battery is large. Although it is possible for a user to manually switch the operation mode, the user needs to pay attention to the operation mode, which results in an extra burden to the user.
Japanese Patent Application Laid-Open No. 2003-116166 discloses, as a system capable of detecting the location of a portable handset without using a GPS, a location detecting system in which a plurality of base units performing near-field radio communication are arranged in a lattice pattern at intervals of a maximum communication distance, so that a portable handset can detect the location of the portable handset by searching for the base units always. According to this location detecting system, a search procedure is repeated in which: the portable handset transmits the ID thereof at the time of finding a base unit; a link is built through authentication given by the base unit; the link is released after the building of the link; and a search for another base unit is performed. Each of the base units herein receives the ID of the portable handset and transmits the received ID to a host computer in this system. Since the location of each base unit is known and is recorded in the host computer, the location of the portable handset can be identified based on the location of the base unit that has communicated with the portable handset having the corresponding ID.